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Old 28th May 2004, 12:38 AM   #1
mjarve is offline mjarve  United States
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Exclamation Fender 6L6 Amp... HELP!!!

A friend is having me replace all the tubes (both pre- and power-amp) in his Fender Twin Reverb. The original power-amp tubes are all miss-matched (two Fenders, a GE, and a Sylvania), and the pre-amp tubes are all original. I began by switching out all the tubes, and prepared to bias it. But the power-amp tubes are running very hot (the plates glow), and there is a horrendous hum and crackle sound when you put it out of stand-by. All the new power-amp tubes are matched Groove Tubes GT6L6GC #5ís. I put the original tubes back in and noticed the GE tube actually had the plate burned through! However it seemed to operate with the original tubes installed, and the hum was lessened (although not gone) and the crackling gone. So what have we got here?

I do not have a lot of experience with tube equipment, so I do not even know where to begin.

Thanks,
Mike
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Old 28th May 2004, 03:33 AM   #2
SY is offline SY  United States
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This is going to take some probing around in the circuitry with a scope and meter. If you're inexperienced with tube circuits, don't even THINK about doing this- the voltages used are lethal, and electrolyic caps can explode. It's time to bring in a technician; you've done what you can do.

Primae non nocere, or however you spell that.
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Old 28th May 2004, 04:51 AM   #3
Jamh is offline Jamh  United States
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I had a similar case with an old Altec tube amp I was restoring. First I put in new tubes and noticed that they were running hot and there was a lot of noise. In my case (your case might be different) the problem was in the capacitors. They had deteriorated. When you used old tubes, because they had lost a lot of their power, they didn't draw very much current from the caps and it seemed to work. But with new tubes the problem became apparent and I had to change the caps to make it work.

Another note. When you change the caps, now you are stressing more the resistors and all parts of the circuit as well. So more problems might pop out.

Please be careful. Even after disconnecting the power cord, the capacitors can carry enough power to seriously injure you. Good luck.
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Old 28th May 2004, 07:50 AM   #4
SHiFTY is offline SHiFTY  New Zealand
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Gah, be damn careful, there might be upwards of 400VDC inside that amp.

That being said- after unplugging the unit, discharging all the caps with a 50 ohm resistor, and removing the tubes:

1) replace any old selenium diode rectifiers with a silicon diode 1n4007 etc
2) replace any burst or leaking electrolytic capacitors (replace all low voltage ones, high voltage ones if necessary)
3) replace all coupling capacitors (they usually range from 0.01- 0.47uF), and are commonly wax paper

Try these things if you are really keen. Try to find out the model number and download the schematic for this amp.
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Old 7th February 2007, 05:22 AM   #5
Nixie is offline Nixie  Canada
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I'm also having a problem with a similar amp, and I'm adding a pointer to another thread where I posted about it: http://www.diyaudio.com/forums/showt...33#post1127133
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Old 7th February 2007, 06:30 AM   #6
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Quote:
Originally posted by Nixie
I'm also having a problem with a similar amp, and I'm adding a pointer to another thread where I posted about it: http://www.diyaudio.com/forums/showt...33#post1127133
Gingertube in that thread gave a reasonable answer. Also, before changing tubes I would recomend to increase a negative bias voltage, it is a precautious measure that often helps to save new tubes, especially when worn-out tubes were biased for the current.
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Old 7th February 2007, 06:45 AM   #7
Nixie is offline Nixie  Canada
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The answer he gave to the other person was that the grids were melted. That obviously cannot be the case here since the problem remained after I replaced ALL the tubes.

I'm not sure how much negative the bias should be. The schematic below shows it should be set to -54.6 V. Also, there's only one adjustment, not separate for both tubes.
http://www.fender.com/support/amp_sc..._Schematic.pdf
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Old 7th February 2007, 11:26 PM   #8
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Mike, are you saying that it is impossible to bias the new output tubes, that even with the bias pot turned all the way to one extreme (the cold end) the new tubes still run too hot?

-- Dave
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Old 8th February 2007, 01:03 AM   #9
Nixie is offline Nixie  Canada
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Dave, Mike posted that in 2004; I'm sture he's solved it by now.
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Old 8th February 2007, 02:32 AM   #10
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The Fender Twin Reverb restoration "bible".
1) Replace the 470 Ohm 2W Carbon Film screen resistors with 1K 5W wire wound (If you check the resistance of the old 470R ones you will measure anywhere between 180R and 850R on intact ones and you will often find ones that fall in half when you unsolder them - that is even though a quick inspection seemed to indicate that they were OK they were actually stuffed (as we say in Tech Talk). If a screen resistor to any output tube is "stuffed" that tube will conduct practically no current and you will get massive hum from inbalance currents in the output tranny.

2) During the step above, when you have the old screen resistors off and before putting in the new ones you have read acess to the 1K5 Grid stoppers, Replace those as well with 1W metal films.
3) I usually replace the entire bias supply chain at this time - all series resistors, the filter cap and thed diode - 70% of all Fender failures I've seen can be traced back to Bias Supply Failures.
OPTIONAL:
While you have clear acess to the output tube sockets consider (seriously) removing the wire link from the cathode pin to chassis on each socket and fitting 10 Ohm 1/2 watt or even 1/4 watt resistors in place of the wire link. This does 2 things: It gives you a place to measure the bias current for each tube. The resistors act as fuses in the event of a bias failure or a tube short which can save your output tranny from damage. This also allows you to actually change some resistors in the bias chain if you are not happy with the idle currents although that generally won't be required (cause twin Reverbs dont have a bias level pot only a bias balance pot).

Once that is done:
With the output tubes NOT fitted and the amp still in standby use your multimeter to check the bias voltage on the Grid 1 pin to cathode pin off EACH output tube socket. NEVER refit the output tubes and switch out of standby until you are 100% happy with this.
The required voltage can be read of the schematics which you can find at Schematic Heaven.

When you are happy with the above fit the output tubes and get the amp up to operating (standby off) and then you can set the balance pot purely by ear - stick your head against the speaker and adjust the balance pot for minimum hum - that meams balance idle currents in the output tranny.

Dashed this off in a hurry but I hope there is something useful there.

Cheers,
Ian
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